Integrators are electronic devices that produce an output signal equal to the time integral of the input signal. These integrators are characterized by the integration rate, and their ability to remember the signal over time. For capacitive integrators, the charging current and the value of the capacitor determine the integration rate, and the leakage current from the capacitor determines the effectiveness of the integrator's memory.
A capacitive integrator utilizes a storage capacitor to store and maintain charge within the integrator circuit. Traditionally, these capacitive integrators are implemented with capacitive negative feedback using an operational amplifier (op-amp) having a gain approaching infinity. However, due to this requirement of high open-loop gain, the op-amp must be carefully compensated to maintain stability. Such compensation is very difficult, especially for applications requiring high-speed processing. In addition, because of the high gain, the op-amp is more susceptible to picking up unwanted noise.
When a capacitive integrator is used in a phase-lock loop circuit, the capacitor is chosen such that the loop is stable over an operating frequency range. If this range is scaled, then the integrating rate of the integrator must be scaled accordingly. Typical integrators with a constant charging current require the value of the capacitor to be scaled inversely by the same factor to achieve the required charging rate. For a fixed-valued capacitor, it must be physically replaced by substitution. This process is inconvenient, expensive, and time-consuming.
It is desirable, therefore, to provide an inexpensive, relatively simple capacitive integrator which does not require an operational amplifier.
It is also desirable to produce a capacitive integrator which can adjust the charging rate without changing the value of the capacitor.